The purpose of this study was to demonstrate the CD11b expression on granulocytes from septic neonates is increased. If so, it will be possible to test the hypothesis that CD11b expression can accurately predict sepsis, and can discriminate infants who are septic from those who are at risk. We found that of the 106 patients who were monitored for bacterial infection five had bacterial infections that were confirmed by cultures. All five of these infants showed high levels of CD11b when they were first admitted to intensive care, but the infants did not show high CRP levels until later, generally one or two days after admission. Two babies had confirmed viral infections and both of them had high CD11b levels on admission, but neither had high CRP lelvels. In 17 babies where infection was suspected because of symptoms, but cultures did not confirm the infection. In 16 of these infants, the CD11b lelvels were high and 15 later showed a high CRP level. Of the remaining 82 infants with no evidence of infection all had low CD11b levels at admission, although 2 later showed high CRP levels. Thus, within 6 hours of birth, the CD11b test correctly predicted the diagnosis of all the babies with confirmed infection and all but one of those with suspected infection. It also correctly excluded all babies who were infection-free.